How To Power Tune Jaguar XK 3.4, 3.8 & 4.2 Litre Engines (SpeedPro series) by Hammill Des

Author:Hammill, Des
Language: eng
Format: epub
Publisher: Veloce Publishing Ltd
Published: 2014-08-30T16:00:00+00:00

It’s a good idea to replace these items with new genuine replacement parts.

Valve collets and keepers

Most of these engines will have done thousands of miles by now and most will still have the original collets and keepers fitted. Caution! - It is strongly recommended that brand new, genuine Jaguar collets and keepers be bought and fitted to any engine that is being rebuilt for high performance. This is a measure to preclude, as much as possible, the failure of one of these items. Failure of these parts is rare but it does happen from time to time. For the modest cost of brand new replacement parts the benefits can be enormous.

Valve springs

Caution! - Always use new valve springs in high performance applications so that the valve spring tension is the maximum possible with that particular spring set. Valve springs lose tension with age and use, so never fit used valve springs to a high performance engine unless they’re relatively new and still make their rated poundage (within 5%).

Caution! - Electronic rev limiters are readily available and should be fitted to all XK engines when they are being used in competition. With standard, but new, valve springs fitted there is not a lot of room for error with regard to engine over speeding. The standard poundage, while being more than adequate for a standard engine does not allow all that much of a safety margin at engine speeds beyond 6000rpm. Fitting a rev limiter is the solution to possible valve damage via engine over speeding (caused by missed gear shifts, and so on), where the engine rpm can rocket. Avoid this situation at all costs.

Standard type

The standard dual valve springs are designed to work with standard valve lift and not significantly more. Taken to the safe limit, this means that standard dual valve springs are only good for a maximum of 0.410in/10.41mm of valve lift. That’s 0.035in/0.889mm more than standard. Caution! - Even though the standard dual valve springs will take slightly more than this, it’s not safe on the basis of valve spring reliability (too near to coil bind). Jaguar valve springs are, of course, of excellent quality, but there are limits to just how much a valve spring can be compressed before spring breakage, through fatigue, can happen.

The fitted height of the outer valve spring in XK cylinder heads is approximately 1.325in/33.65mm. The fitted height of the inner valve spring is approximately 1.250in/31.75mm. The coil bind height of later valve springs (in combination) is approximately 0.875in/22.22mm. This means that the valve springs in combination are coil bound at 0.450in/11.43mm of valve lift. This is why the maximum recommended valve lift using standard valve springs is 0.410in/10.41mm as it leaves a total clearance between the coils of 0.040in/1mm.

The seated tension of later XK valves springs, for example, is approximately 70-80 pounds/31.75-36.28kg. The (valve) fully open pressure is approximately 130-140 pounds/58.96-63.50kg at 0.375in/9.525mm of valve lift (outer valve spring compressed to approximately 0.950in/24.13mm). These later standard dual valve springs are quite sufficient for all high performance engines with camshafts featuring up to 0.

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